Energy & Fuels 2007, 21, 1707-1711
1707
Sodium Borohydride Synthesis by Reaction of Na2O Contained Sodium Borate with Al and Hydrogen Bin Hong Liu,† Zhou Peng Li,*,† Jing Ke Zhu,† N. Morigasaki,‡ and S. Suda‡ College of Materials Science and Chemical Engineering, Zhejiang UniVersity, Hangzhou 310027, P. R. China, and Materials & Energy Research Institute Tokyo, Ltd., Sawarabi-Daira Tateshinachuo-Kogen, Kitayama, Chino-shi, Nagano, Japan ReceiVed NoVember 1, 2006. ReVised Manuscript ReceiVed March 8, 2007
In this paper, sequential experiments were designed to explore the possibility of sodium borohydride synthesis by reaction of sodium borates with Al and hydrogen. It was found that NaBH4 cannot be formed by reaction of NaBO2 with Al and hydrogen but can be formed by reaction of Na4B2O5 with Al and hydrogen. NaBH4 formation was confirmed by X-ray diffraction analyses and morphology observations. It was found that Na2O in Na4B2O5 played an important role in NaBH4 formation due to the depression of Al2O3 formation. The highest sodium borohydride yield (65.8%) was obtained by reaction of a mixture of Na4B2O5, NaBO2, and Na2O with Al and hydrogen. However, NaBH4 formation was dependent upon the physical state of sodium borates. When sodium borates melted, NaBH4 formation was retarded due to the difficulty of hydrogen diffusion to Al surfaces.
2Mg + 2H2 + NaBO2 ) NaBH4 + 2MgO
1. Introduction Generally, borohydrides are used as reducing agents to make hydrocortisone, prednisone, prednisolone, and corticosteroids. They are also used in vitamin A synthesis, chemical plating, and so forth. Recently, they have been attracting more attention as hydrogen storage materials for hydrogen generation or as a fuel for the direct borohydride fuel cell (DBFC). Many papers on hydrogen generation and the DBFC have been published. However, there are only a few papers related to recycling of the spent fuel.1-5 The most common synthesis method of sodium borohydride was invented by Schlesinger et al.,6 as shown below:
4NaH + B(OCH3)3 ) NaBH4 + 3NaOCH3
(1)
hydride7
and magnesium Besides sodium hydride, calcium hydride1-3 can react with NaBO2 to form NaBH4 by the following reaction:
2Ca(Mg)H2 + NaBO2 ) NaBH4 + 2Ca(Mg)O
(2)
It was reported that sodium borohydride can be formed by reaction of NaBO2 with Mg and hydrogen around 723 K through the following reaction:2,4,5 * Corresponding author tel.: 86-571-87951977; fax: 86-571-87943149; e-mail:
[email protected]. † Zhejiang University. ‡ Materials & Energy Research Institute Tokyo, Ltd. (1) Li, Z. P.; Morigasaki, N.; Liu, B. H.; Suda, S. J. Alloys Compd. 2003, 349 (1), 232. (2) Kojima, Y.; Haga, T. Int. J. Hydrogen Energy 2003, 28, 989. (3) Li, Z. P.; Liu, B. H.; Morigasaki, N.; Suda, S. J. Alloys Compd. 2003, 354 (1/2), 243. (4) Suda, S.; Iwase, Y.; Morigasaki, N.; Li, Z.-P. AdV. Mater. Energy ConVers. II, Proc. Symp. 2nd 2004, 123. (5) Suda, S.; Morigasaki, N.; Iwase, Y.; Li, Z. P. J. Alloys Compd. 2005, 404-406, 643. (6) Schlesinger, H. I.; Brown, H. C.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra, H. R.; Hyde, E. K. J. Am. Chem. Soc. 1953, 75, 215. (7) Goerrig, D.; Schabacher, W.; Schubert, F. DBP 1036222, 1956.
(3)
Compared with Ca, Mg, and Na, Al not only is a cheaper material but also can offer more electrons when reducing sodium borates to form sodium borohydride. Al would be a favorable raw material for NaBH4 synthesis. In this paper, sequential experiments were planned to explore the possibility of sodium borohydride formation by reaction of sodium borate with Al and hydrogen. The mechanism was studied on the basis of X-ray diffraction (XRD) analyses, morphology observations, and differential scanning calorimetry (DSC) analyses. 2. Experimental Section In this research, Al powder (purity: 99.9%,
